Sunday, February 27, 2011

Evolution - 7th Grade

EVOLUTION

Skills to Be Taught @ Evolution

3.D.6 EVOLUTION

3.D.6.1. Explain that in any particular environment, the growth and survival of organisms and species depend on the physical conditions.

a. Cite examples and describe that small differences between parents and offspring can accumulate (through selective breeding) in successive generations so that descendants are very different from their ancestors.

b. Explain that in all environments-freshwater, marine, forest, desert, grassland, mountain, and others-organisms with similar needs may compete with one another for resources, including food, space, water, air, and shelter.

c. Explain that in any particular environment individual organisms with certain traits are more likely than others to survive and have offspring.

d. Explain, with examples, ways that people control some characteristics of plants and animals they raise by selective breeding.

e. Describe ways in which changes in environmental conditions can affect the survival of individual organisms and entire species.

f. Describe how sediments of sand and smaller particles (sometimes containing the remains of organisms) are gradually buried and are cemented together by dissolved minerals to form solid rock; and describe that such fossils provide evidence for the long history of changing life forms whose remains are found in the rocks.

1 In 1831 the HMS Beagle left England on a journey to chart the coastline of South America. The captain hired Charles Darwin to be the ship's naturalist. Darwin, only twenty-two years old at the time, was more interested in science than the career as a clergyman that his father had planned for him. Darwin's job on the Beagle was to take notes about the living things he saw throughout the voyage.

2 The plants and animals Darwin saw in South America were very different from those in England. He wondered about the diversity of the animals and their environments. Could one of those things affect the other?

3 The Beagle's voyage was intended to last for two years. Instead, it was five years before Darwin returned to England. During those five years, he spent about two-thirds of his time on land observing plants, animals, and fossils. Darwin was amazed at the diversity of the living things he saw. The fossils, too, puzzled him. Darwin saw fossils of animals like the sloths he saw living in the trees. However, the fossils were much bigger than any of the living animals Darwin saw. Darwin wondered what had happened to the larger creatures of the past.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading above.

4 When they reached the Galapagos Islands, Darwin saw even more diversity in the animal species living there. The Galapagos are a group of small islands about six hundred miles from the west coast of Ecuador. Darwin saw giant tortoises, some with a domed shell, and some with a saddleback shell. He even saw some with shells that were in between.

5 Some of the plants and animals on the islands were similar to those on the mainland, but there were differences. Cormorants, large sea birds, lived in both places. The ones that lived on the Galapagos Islands were unable to fly. Darwin inferred that some of the plants and animal species had come to the islands from the mainland. After many generations of the species reproduced on the islands, their offspring had different traits than the species on the mainland.

6 Darwin even noticed there were differences among species of the same animals that lived on different islands. The tortoises had differently shaped shells depending on which island they lived. Local people told Darwin they could identify the island by looking at the tortoises' shells living there.

7 The finches living on different islands had noticeable differences, too. The most striking differences were the size and shapes of the birds' beaks. The birds' beaks were well suited to the type of food the birds ate. This adaptation helped the birds survive and reproduce in their own environments. A gradual change in a species over time is called evolution.

8 Darwin knew farmers used selective breeding to produce sheep and other domestic animals with desired traits. By allowing only the animals with desired traits to reproduce, farmers were able to breed better farm animals. He wondered if a similar process happened in nature.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading above.

9 After returning to England, Darwin did not publish his findings for twenty years. Darwin's theory of evolution by natural selection has become one of the most important, and most controversial, scientific theories of all time.

saddleback selective natural career clergyman similar which controversial

adaptation

The Galapagos Islands off of South America

Directions: Copy this article in your journal and Fill in each blank with the word that best completes the reading comprehension.

In 1831 the HMS Beagle left England on a journey to chart the coastline of South America. The captain hired Charles Darwin to be the ship's (1) _______________________ . Darwin, only twenty-two years old at the time, was more interested in science than the (2) _______________________ as a (3) _______________________ that his father had planned for him. Darwin's job on the Beagle was to take notes about the living things he saw (4) _______________________ the voyage.

The plants and animals Darwin saw in South America were very different from those in England. He wondered about the diversity of the animals and their environments. Could one of those things affect the other?

The Beagle's voyage was intended to last for two years. Instead, it was five years before Darwin returned to England. During those five years, he spent about two-thirds of his time on land observing plants, animals, and fossils. Darwin was amazed at the diversity of the living things he saw. The fossils, too, puzzled him. Darwin saw fossils of animals like the sloths he saw living in the trees. However, the fossils were much bigger than any of the living animals Darwin saw. Darwin wondered what had happened to the larger creatures of the past.

When they reached the Galapagos Islands, Darwin saw even more diversity in the animal species living there. The Galapagos are a group of small islands about six hundred miles from the west coast of Ecuador. Darwin saw giant tortoises, some with a domed shell, and some with a (5) _______________________ shell. He even saw some with shells that were in between.

Some of the plants and animals on the islands were similar to those on the mainland, but there were differences. Cormorants, large sea birds, lived in both places. The ones that lived on the Galapagos Islands were (6) _______________________ to fly. Darwin inferred that some of the plants and animal species had come to the islands from the mainland. After many generations of the species reproduced on the islands, their offspring had different traits than the species on the mainland.

Darwin even noticed there were differences among species of the same animals that lived on different islands. The tortoises had differently shaped shells depending on (7) _______________________ island they lived. Local people told Darwin they could identify the island by looking at the tortoises' shells living there.

The finches living on different islands had noticeable differences, too. The most (8) _______________________ differences were the size and shapes of the birds' beaks. The birds' beaks were well suited to the type of food the birds ate. This (9) _______________________ helped the birds survive and reproduce in their own environments. A gradual change in a species over time is called evolution.

Darwin knew farmers used (10) _______________________ breeding to (11) _______________________ sheep and other domestic animals with desired traits. By allowing only the animals with desired traits to reproduce, farmers were able to breed better farm animals. He wondered if a (12) _______________________ (13) _______________________ happened in nature.

After returning to England, Darwin did not publish his findings for twenty years. Darwin's theory of evolution by (14) _______________________ selection has become one of the most important, and most (15) _______________________ , scientific theories of all time.

This geographic separation from other of their species seems to be what made different species evolve on the Galapagos Islands. The 600 mile separation of the islands from the mainland may have led to those animals evolving into new species.

Characteristics of Organisms

1.Individuals of the same kind differ in their characteristics, and sometimes the differences give individuals an advantage in surviving and reproducing.

2.The characteristics of organisms affect their ability to survive and reproduce.

There are ways in which organisms in one habitat differ from those in another habitat and thesedifferences help them to survive and reproduce.

New species might form when a group of individuals remains separated from the rest of its species long enough to evolve different traits. note Kaibab squirrel in Arizona.

ASSIGNMENT=

Please look up the Kaibab squirrel in Arizona on the internet and do a report @ how it acquired different traits than other squirrels.

Please write a 2 + paragraph report and cut and paste some pictures that are examples. Then send it as your comment to the blog.

Continental drift causing Pangaea to separate into other continents. This created isolation and species evolve independently of each other

3.

ASSIGNMENT- Please watch this video, and explain what it is showing in your journal.

Theory of Evolution

•The theory of evolution says that species change over time. Did you ever wonder how new species evolve? Natural selection seems to explain how variations can lead to changes in a species. Geographic isolation seems to be one of the main ways this happens.

ASSIGNMENT- Please watch this video, and explain what it is showing in your journal.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading below.

•Overproduction happens when species reproduce many more offspring than can possibly survive. The world has limited resources. Many species create many more offspring than there are resources like food, water, and living space to support them. This creates a struggle to survive for the offspring. Those who are better able to survive then pass on their genes to their offspring. This is the process of natural selection.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading above.

When did humans evolve? Who are our ancestors? Why did we evolve? The phony Piltdown Man.

At the turn of the century, scientists could only dream about finding the answers to these questions. The fossilized remains of a species known as Neanderthal had been found, and there was a primitive, human-like skull that had been discovered in Indonesia.
Beyond that, there was little hard evidence to work with.

With the 1912 "discovery" of the Piltdown Man, the study of human evolution was sent down a wrong track. When the phony Piltdown Man, with its human skull and orangutan's jaw, was finally exposed in 1953, the pieces of the great puzzle began to fall into place. With the experts' opinions no longer skewed, the relationship between the real fossils started to make sense.

There have been many discoveries, and much has been learned about the human odyssey over the past few decades. Many questions, however, still remain.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading above.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading above.

1.Individuals of the same kind differ in their characteristics, and sometimes the differences give individuals an advantage in surviving and reproducing.

2.The characteristics of organisms affect their ability to survive and reproduce.

3.There are ways in which organisms in one habitat differ from those in another habitat and these differences. An example of this is the finch birds in the Galapagos Islands

The Finches on the Galapagos Islands

The finches ( a kind of bird) on the Galapagos Islands eat a variety of foods, including seeds, nectar from fruit and insects. Charles Darwin noted variation (differences) in the length and width of the finches' beaks based on the finches' diet.

Assignment=Please copy this graphic organizer and complete it in your journal from the reading above.

Copy and answer in your JournalAssignment=Which of the following explanations for the situation of the finches on the GALAPAGOS ISLANDS explains how natural selection worked in the case of these birds? (pick one)

a) Many finches with different shaped beaks migrated from South America (600 miles across the ocean) and naturally settled in areas where they could eat the availablke food.
b) One kind of finch migrated to the Galapagos, and the beak of each individual finch chganged to match it's food source during it's lifetime.
c) Finch beaks tend to naturally match the food in the environments in which those finches originated .
d) Variations or changes in finches' beak sizes and shapes allowed those finches with beaks suited to the food available to them to survive and reproduce.

Mechanisms: The Processes of Evolution

Evolution is the process by which modern organisms have descended from ancient ancestors. Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life—but exactly how does it work?

Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. This section examines the mechanisms of evolution focusing on:

Descent and the genetic differences that are heritable and passed on to the next generation;

The random nature of genetic drift and the effects of a reduction in genetic variation;

How variation, differential reproduction, and heredity result in evolution by natural selection; and

How different species can affect each other’s evolution throughcoevolution.

Human Evolution (1 of 2)

Evolutionary biologists are interested in understanding how humans fit into the history of life and how the processes of evolution have shaped us. Much scientific effort goes into studying human evolution, and as a result, our understanding of this area is moving forward rapidly, as new evidence emerges and hypotheses are tested, confirmed, discarded, or modified.

The location of our very own twig: Humans on the tree of life
This tree is based on morphological and genetic data. Chimpanzees and humans form a clade with DNA sequences that differ by only 1%1. This genetic similarity made it hard to figure out exactly how these two primates are related, but recent genetic studies have strongly suggested that chimpanzees and humans are each other’s closest living relative.2

Human Evolution (2 of 2)

How did humans evolve?
About six million years ago in Africa1, the chimpanzee lineage and our own split. What happened to us after that split? The hominid lineage did not march in a straight line to Homo sapiens. Instead, the early hominid lineage gave rise to many other (now extinct) hominids. Examining the fossils, the artifacts, and even the DNA of these relatives has helped us understand how this complex hominid tree evolved, and how modern humans came to exist.

Here are some of the important events in human history, with approximate dates, which reflect the evidence currently available:

4) 2.5 mya: Some hominids made tools by chipping stones to form a cutting edge. There were perhaps four or more species of hominid living in Africa.

5) 2 mya: The first members of the Homo clade, with their relatively large brains, lived in Africa.

6) 1.5 mya: Hand axes were used. Also, hominids had spread out of Africa and into much of Asia and Europe. These hominids included the ancestors of Neanderthals (Homo neanderthalensis) in Europe andHomo erectus in Asia.

7) 100,000 years ago: Human brains reached more or less the current range of sizes. Early Homo sapiens lived in Africa. At the same time, Homo neanderthalensis and Homo erectus lived in other parts of the Old World.

Locate the Galapagos Islands on a world map so that students can clearly see where they are located (in the Pacific Ocean about 600 miles, or 970 kilometers, west of Ecuador).

2.

Show students pictures of the Galapagos and of some of the unusual animals that live there (e.g., giant tortoises, iguanas, blue-footed boobies).

3.

Review with your students what they know about the Galapagos Islands and their significance. Be sure they know that Charles Darwin visited the islands in 1835 and studied the animal life there. He was particularly interested to observe how animal species had adapted, over a long period of time, to conditions in an isolated part of the world.

4.

On the chalkboard, write the terms endemic and introduced. Explain that endemic species of animals or plants are those that are native to certain geographical areas and restricted to those areas. Tell students that the animals in the pictures they have just seen are endemic to the Galapagos Islands. Then explain that introduced species are those brought into an area where they would not naturally exist.

5.

Discuss with the class the possible effects of introduced species on endemic species, making the point that introduced species can actually endanger species endemic to a particular location. For example, certain introduced plants, also called "invasives," can take over an area and crowd or choke out native plants. Another example to consider is an introduced animal species that preys on endemic animal species or occupies the habitat of endemic species.

6.

Go on to explain that, in the Galapagos Islands, introduced species are presently endangering endemic species.

7.

Using research materials such as field guides, help students identify species of plants and animals endemic to your area. Help students find out what conditions might have allowed some plants to become endemic. (Your state department of agriculture can be a help.)

8.

Have students list as many endemic plants as they can.

9.

With the class, use the research students have done to plan a garden of plants endemic to your area. Plans should include how to keep introduced, or invasive, species out of the garden.

10.

As a class project, create an endemic garden on the school grounds or on a nearby available plot of ground.

How did Darwin's work aboard the Beagle differ from the way scientists today work aboard the Steward Johnson?

2.

Why are island populations so important in the study of animal adaptation?

3.

What significant evidence do sea mounts provide about the history of the animals located today on the Galapagos Islands?

4.

What adaptations must deep water organisms make as they are brought up for study from the deep sea environment?

5.

What is meant by evolution through natural selection?

6.

Many people believe, incorrectly, that the phrase "survival of the fittest" means only the strong survive. What does "survival of the fittest" really mean?

Cool Lesson Plan to try in your class

TITLE: Animal Survival
AUTHOR: Jeffrey Kimber, McGill Elem., Ely, NV
GRADE LEVEL/SUBJECT: science
OVERVIEW: All children love animals but often do not
understand why, in the course of nature, they must die to
allow others to survive.
PURPOSE: The purpose of this lesson is to demonstrate to
the class that it is not cruel or unfair when animals die.
It is only natural that the strongest of each species
survive while the weak perish.
OBJECTIVES: The students will demonstrate an understanding
of why some animal perish while others survive. This will be
evaluated through classroom discussion and a simple quiz.
RESOURCES/MATERIALS: handkerchiefs for blindfolds, scarves
for tying up a broken leg, and chips or markers to be used
for food, video - "The Lions of Africa."
ACTIVITIES AND PROCEDURES: This lesson is designed as more
of a game than a sit down experiment. The children become
the animals for the lesson. Any animal can be chosen as
long as the children are all the same animal. The object of
the game is survival. To survive, each child must gather
enough food chips to live. Those who don't will perish. To
make the lesson effective, not all of the children can be
healthy animal. This should be explained to the children
that in nature, not all animals are healthy. Some of the
children should be blindfolded to make them blind. Others
should have other disabilities such as a broken leg which
cannot be used, a broken back which halts the use of both
back legs, etc. It aids in the children's understanding if
the teacher tells the students how the animal got his
disability using real life situations.
The actual game begins with spreading the food chips
around the floor of the classroom. All of the animal start
in one particular spot. When the teacher tells the children
to start, they crawl around the room gathering as many food
chips as they can in the time allotted. This time allowed
will depend on the size of the class. When time is called,
the animals step gathering and return to their seats. The
teacher then writes on the board how much food they needed
to survive and for how long. For example, a deer that
gathered 30 food chips is healthy for the next year, whereas
a deer who gathered 20 food chips may be healthy for only
six months. A deer who only gathered 10 food chips or less
will probably only live for another two or three months.
This part of the lesson is followed by classroom discussion
of what happened to the deer in our forest. The children
will note which deer were the first to perish, usually the
lame deer or the very old or sick. With some teacher
assistance, the children will internalize the activity to
relate to when their cat had kittens and one died, etc.
This is a good lesson for those classrooms which have a
class pet who has died also.
TYING IT ALL TOGETHER: After classroom discussion, a good
culminating activity is to show the video "The Lions of
Africa." It discusses a pride of lions surviving a drought
in Africa and what becomes of them. It is a very factual
and sometimes graphic video and should be previewed by the
teacher for lower elementary grades.

POST TEST - Copy and put in your Journal

§How did Charles Darwin explain the differences between species on the Galapagos Islands and on mainland South America?

§What observations led Charles Darwin to propose his theory of evolution?

§How does natural selection lead to evolution?

§How fast does evolution occur?

§How do new species form?

§What role does geographic isolation play in evolution?

§How do most fossils form?

§How can scientists determine a fossil’s age?

§What is the fossil record?

§What do fossils reveal?

§What is a half-life?

§How are species classified?

§What evidence from modern day organisms can help scientists determine evolutionary relationships among groups?

§What information do homologous structures reveal?

WATCH THESE VIDEOs, AND TAKE 10-20 NOTES for Each Video IN YOUR JOURNAL ABOUT WHAT YOU LEARNED.

)

Darwin

Charles Darwin was a British scientist who lived in the nineteenth century. He was born in England in 1809. Darwin�s concept of natural selection changed the way people thought about the development of life, especially the evolution of man.

Darwin went to the University of Edinburgh to study medicine, but he dropped out before finishing and went instead to the University of Cambridge to become a preacher. While he was there, he met two men that had an important influence on his future. One was Adam Sedgwick, a geologist, and the other was John Stevens Henslow, a scientist who studied nature. After Darwin graduated from Cambridge, Henslow convinced him to come aboard the ship HMS Beagle as an unpaid assistant. They would travel around the world observing nature. Darwin was impressed with nature. He was amazed by the way natural forces had shaped the surface of the earth.

OBJECTIVES

Recognize that adaptations may include variations in structures, behaviors, or physiology, such as spiny leaves on a cactus, birdcalls, and antibiotic resistant bacteria.

An organism’s living and non-living surroundings make up its environment. When there is a gradual change in the environment, populations with sufficient variation among their members might be able to adapt to the change. In some populations, there is a great deal of variety among individuals. When individuals with the most favorable traits reproduce, their offspring receive those traits and can pass them on to their offspring. When the environment is stable, there may be very little change in a population. Some species, such as horseshoe crabs, have earned the nickname “living fossils” because they have not changed for millions of years. But when there is a sudden change in the environment — a colder climate, an epidemic, or a new predator — many individuals within a population may be killed. If the environmental change is on a large scale, it can affect an entire species. Species that cannot adapt to environmental changes may die out completely and become extinct. Those organisms within a species that have favorable traits will survive and reproduce. Gradually, the organisms within the population may look and behave differently. At some point, the changes in a population are so significant that a new species has formed. This change is called evolution. Evolution is responsible for the vast diversity of life on Earth.

Working in groups, have students read the following articles dealing with gradual and rapid environmental changes. Each student in the group will read one article and share its contents with the rest of the group.

Students produce a presentation of the key points of their findings (poster, PowerPoint, brochure, news report, etc.) describing how gradual climatic and sudden changes in the environment can affect the survival of organisms and populations.

/instruction/lessons/science/grade8/xml/3D1a.xml

Resources for Objective 3.D.1.a:

ASSIGNMENT=Cooperative activity

Working in groups, have students read the following articles dealing with gradual and rapid

environmental changes. Each student in the group will read one article and share its

Students produce a presentation of the key points of their findings (poster, PowerPoint,

brochure, news report, etc.) describing how gradual climatic and sudden changes in the

environment can affect the survival of organisms and populations.

Science Toolkit: Grade 8 Objective 3.D.1.a

Content

Adaptations are characteristics that help an organism survive and reproduce in a particular environment. The adaptation can be a physical or behavioral characteristic.

A zebra’s stripes are a physical adaptation. A zebra is able to blend in well with its environment and hide from predators. The stripes make it difficult for a lion to distinguish an individual zebra walking through tall grass. Because of its green color, a red-eyed tree frog is able to hide under leaves during the day. It comes out at night in search of insects. The frog’s red eyes and bright coloring may distract predators and allow it time to escape.

Cacti grow and live in very hot and arid environments. In order to survive in this type of environment, they store water in enlarged stems. The spines on the stem are actually leaves adapted to prevent water loss by reducing their surface area. They also protect the plant from herbivores. Cacti also have a thick, tough covering that reduces water loss from the plant.

Behavioral adaptations in birds include calls and songs. Bird calls among members of a species or of different species may serve to alert other birds to danger or attract them to an area. The quality of a bird song may be an indicator of the fitness of the bird; the better the song, the more likely bird will be to attract a mate.

Some bacteria have developed a resistance to antibiotics. Because of natural variations within the bacteria population, some survive and pass on the resistance to the next generations. Bacteria reproduce through the process of binary fission, rapidly increasing the number of bacteria resistant to antibiotics in a short period of time.

In any habitat, food is limited and the types of foods available may vary. Animals that are better adapted to take advantage of available foods will fare better than those who are less well adapted, and thus live to pass on their genes to the next generation. While this concept seems rather obvious, it is essential that each student fully grasp its significance. Understanding the idea of adaptive advantage opens the door to understanding populations in ecosystems as well as the process of evolution.

Explore this link for additional information on the topics covered in this lesson:

Have students select either a spoon, tweezer, binder clip or pair of scissors, and a plastic cup and then sit quietly in a large circle.

Explain to them that they are now birds. They are very hungry birds. They can only eat with the implement they have selected and they can only use that implement for eating. The cup represents their stomach. It must remain upright at all times. They must hold their beak in one hand and their stomach in the other. They can only place food in their stomachs with their beaks.

Explain to them that certain food items will be placed in the feeding area (middle of the circle but spread out evenly towards the students/birds). When you say “go” they are to collect as much food and place it in their stomachs as possible until you say “stop.”

Take one of the food items (paper clip beetles) and distribute the clips within the feeding area. Say “go” and allow birds to feed for 1–2 minutes or until all of the food is gone. [NOTE: Depending upon your students you may need to caution about behavior. Even adults doing this activity become a bit more aggressive as the activity proceeds, but obviously safety is foremost. If some students are not responsible enough for this, have them be observers and take notes on the birds’ behaviors.]

Once you have said “stop” have students empty their stomachs and count the contents. Hand each a Recording Sheet to fill in. Have them return all food items.

Repeat this activity using each of the other food items (toothpick twigs, rubber band worms, macaroni munchies). By the end of the activity, each of the students should have completed his/her row for the beak type and filled in the total amount of food.

Pause for a class discussion:
a) What did you notice about your feeding abilities?
b) Did everyone with your type of beak have the same success rate with the same foods? Why or why not?
c) What did you notice about your behavior and the behavior of others?

Examine the data: Tally up the class totals for each of the beak types in a grid on the board. Have the students create bar graphs that represent the class total for each of the beak and food types. This can be started in class and continued for homework.

Second hour

When all of the graphs have been completed, have students pick up their beaks and stomachs once again and return to their circle. Explain that obviously most habitats have more than one kind of food available. Ask: What will your strategy be if all of the food types are available?

Spread out all of the materials into the feeding circle. Allow about 4 minutes for feeding. Gather the data and have students help to sort out the food items once again for clean up.

Again continue with the class discussion. What were your strategies? How was this different from the previous eating experiences?

Assess student understanding by posing this question: What would happen if all the bird types we have been working with flew to an island where no birds had been before and the only food type available was macaroni munchies. Which bird beak type would be mot likely to be successful? Explain your answer.

Extensions:

Give a food value for each of the food types. Would this change your feeding strategies? How much more of one type of food would you have to eat to equal only one of something else, etc. What would happen if there was a change in the environmental conditions (drought, etc) causing the loss of one of the food items. What would happen to the bird populations?

Human Evolution

—By Karen Barss

The story of human evolution began in Africa about six million years ago and it describes the very long process that our ancestors went through to ultimately become modern humans. This process has been uncovered by studying fossils and understanding the underlying theory of evolution, and while new fossils are uncovered every decade revealing new chapters, scientists agree about the basic story.

What Is Evolution?

Evolution means the changes that occur in a population over time. In this definition, a “population” means a group of the same species that share a specific location and habitat. Evolutionary changes always occur on the genetic level. In other words, evolution is a process that results in changes that are passed on or inherited from generation to generation. It does not, for example, describe how people can change their muscle mass by lifting weights.
When successful, these genetic changes or adaptations, which happen when genes mutate and/or combine in different ways during reproduction, help organisms survive, reproduce, and raise offspring. Some individuals inherit characteristics that make them more successful at surviving and having babies. These advantageous characteristics tend to appear more frequently in the population (because those individuals with less advantageous characteristics are more likely to die without reproducing), and over time these changes become common throughout that population, ultimately leading to new species.

The Tree of Life

Biological evolution explains the way all living things evolved over billions of years from a single common ancestor. This concept is often illustrated by the so-called tree of life. Every branch on the tree represents a species. The fork separating one species from another represents the common ancestor that each pair of species shared. So ultimately, all life is interconnected, but any two species may be separated by millions or even billions of years of evolution.

Only a Theory?

Some people dismiss evolution as “just a theory.” Evolution is in fact a theory, a scientific theory. In everyday use, the word theory often means a guess or a rough idea: “My theory is…” “I have a theory about that.” But among scientists, the word has an entirely different meaning. In science, a theory is an overarching explanation used to describe some aspect of the natural world that is supported by overwhelming evidence.
Other scientific theories include cell theory, which says that all living things are made up of cells, and heliocentric theory, which says the earth revolves around the sun instead of the other way around.

The Relationship between Apes and Humans

Since scientists developed the ability to decode the genome and compare the genetic makeup of species, some people have been stunned to learn that about 98.5% of the genes in people and chimpanzees are identical. This finding means chimps are the closest living biological relatives to humans, but it does not mean that humans evolved from chimps. What it does indicate is that humans share a common ancestor with modern African apes (i.e., gorillas and chimpanzees), making us very, very distant cousins. We are therefore related to these other living primates, but we did not descend from them.
Modern humans differ from apes in many significant ways. Human brains are larger and more complex; people have elaborate forms of communication and culture; and people habitually walk upright, can manipulate very small objects, and can speak.

Our Common Ancestor

Most scientists believe our common ancestor existed 5 to 8 million years ago. Then two species broke off into separate lineages, one ultimately evolving into gorillas and chimps, the other evolving into early humans called hominids. In the millions of years that followed, at least a dozen different species of humanlike creatures have existed, reflected in the fossil discoveries of paleoanthropologists, although many of these species are close relatives but not actual ancestors of modern humans.
In fact, the fossil record does not represent a straight line of ancestry at all; many of these early hominids left no descendents and simply died out. Still others are most likely direct ancestors of modern humans orHomo sapiens. While scientists still do not know the total number of hominid species that existed, because new fossils are discovered every decade, the story of human evolution becomes clearer all the time.

What about the Missing Link?

The idea of a missing link has persisted, but it is not actually a scientific term. In the popular imagination, this missing link would be the fossil of our common ancestor. While scientists agree on the concept of a common ancestor, deciding which fossil represents that actual species is challenging if not impossible, given that the fossil record will never be 100% complete. Also, the word implies that evolution is a straight chain of events, when in fact the sequence of evolution is much more complicated.

The Fossil Record

Fossils are the remains or impressions of living things hardened in rock. All living organisms have not been preserved in the fossil record; in fact, most have not because very specific conditions must exist in order to create fossils. Even so, the fossil record provides a fairly good outline of human evolutionary history.
The earliest humans were found in Africa, which is where much of human evolution occurred. The fossils of these early hominids, which lived 2 to 6 million years ago, all come from that continent. Most scientists believe early humans migrated out of Africa into Asia between 2 million and 1.7 million years ago, entering Europe some time within the past 1 million years. What follows are some highlights of the early human species that have been identified by scientists to date.

Australopithecines

An African apelike species evolved probably around 6 million years ago with two skeletal characteristics that set it apart from apes: small canine teeth (the teeth on either side of the four front teeth) compared to the long canines found in almost all other primates, and, most importantly, bipedalism or walking on two legs as the primary mode of locomotion.
The name australopithecine means “southern ape,” in reference to South Africa where the first known fossils were found. Many more australopith fossils have been found in theGreat Rift Valley in eastern Africa, in countries including Ethiopia, Tanzania, Kenya, and Chad.
The very early years of the transition from ape to human, from 6 million to 4 million years ago, is poorly documented in the fossil record, but those fossils that have been discovered document the most primitive combinations of ape and human features.
Fossils from different early australopith species that lived between 4 million and 2 million years ago show a variety of adaptations that mark this transition much more clearly. Among the genera that are included in early australopith species areSahelanthropus, Orrorin, and Aripithecus; a species of the genus Kenyanthropus; and four species of the genus Australopithecus.
Probably the best-known australopith specimen is “Lucy,” the partial skeleton of a female discovered in 1974 in Hadar, Ethiopia. Lucy belongs to a species, Australopithicus afarensis, which thrived in eastern Africa between 3.9 million and 3 million years ago. Scientists have found several hundred A. afarensis fossils in Hadar. Lucy lived 3.2 million years ago.
Another very exciting A. afarensis site was discovered in northern Tanzania at Laetoli. In addition to fossilized bones of A. afarensis, researchers in 1978 discovered trails of bipedal human footprints preserved in hardened volcanic ash over 3 million years ago. The footprints provided irrefutable evidence that australopiths regularly walked upright.
By about 2.7 million years ago, so-called robust australopiths (in contrast to the earlier,gracile forms) had evolved, with wide molars and premolars and a facial structure that indicate that these robust australopiths chewed their food, primarily tough, fibrous plants, powerfully and for long periods. Several robust species have been identified, and the last robust australopiths died out about 1.4 million years ago.

The Genus Homo

The genus Homo first evolved at least 2.3 million to 2.5 million years ago. The most significant difference between members of this genus and australopiths, with which they overlapped, was their significantly larger brains (about 30 percent larger, though still small compared to modern humans).
Scientists divide the evolution of the modern human genus into three rough periods: early, middle, and late. Species of early Homo, among them Homo habilis, resembled australopiths in many distinct ways, but they had smaller teeth and jaws, more modern-looking feet, and hands capable of making tools. They probably lived from between 2.5 or 2.3 million and 1.6 million years ago.
The middle Homo species, including Homo erectus, evolved anatomically to be more similar to modern humans but their brains were relatively small (though bigger than australopiths). They probably overlapped with earlier Homo species, as they developed perhaps between 2 million and 1.8 million years ago. Homo erectus was a very successful species of the middle period; fossils have been found throughout Africa, Europe, and much of Asia, and the species may have survived for more than 1.5 million years.
The final transition, from the middle to late periods, happened about 200,000 years ago. Late Homo species, including Neanderthals and Homo sapiens, evolved large and complex brains, leading eventually to language, and developed culture as an increasingly important aspect of human life.

Homo sapiens

Scientists have dated the oldest known fossils with skeletal features typical of modern humans from 195,000 years ago. Early anatomically modern Homo sapiens fossils have come from sites in Sudan, Ethiopia, South Africa, and Israel. Many scientists have therefore concluded that modern Homo sapiens evolved in Africa and began spreading to other parts of the world 90,000 years ago or a little earlier, although whether, how, why, and when this happened is still in dispute. And it was not until about 40,000 years ago that anatomically modern humans,Homo sapiens sapiens, emerged. Since that time, human evolution has been primarily cultural as opposed to biological.

Putting Human Evolution in Perspective

Humans have existed for only a tiny fraction of Earth’s history. Scientists believe Earth itself is approximately 4.55 billion years old. The oldest known fossils are about 3.5 billion years old, although some scientists have discovered evidence that life may have begun nearly 4 billion years ago. Dinosaurs walked Earth between 230 and 65 million years ago. The oldest known humanlike fossil has been dated at 4.4 million years old, although another species, not yet confirmed as a hominid, has been dated at about 6 million years old. As mentioned earlier, scientists estimate that the earliest hominid species diverged from the ape lineage between 5 and 8 million years ago. And yet, the species to which we belong, Homo sapiens sapiens, is only about 40,000 years old.See also:Historical Development and Mechanisms of Evolution and Natural Selection,Microevolution and Macroevolution, Origin of Life, and Origin of Prokaryotes and Eukaryotes.

Science Grade 8 Objective 3.D.1.e

Activity

Have the students complete the web activity “All in the Family,” available at

http://www.pbs.org/wgbh/evolution/change/family/.

Discussion Questions:

1. Why are physical features such as size or shape misleading when it comes to

evolution?

2. Explain the statement, “All living things are related to each other like the branches on